...gems like wavy glass. Wavy because it was cheaper back in the day, now it's expensive.
That's the second claim about wavy glass in this thread, and this one is at least partly true. The other claim was that the glass is wavy because it flows over time, which implies it was once perfectly planar and clear, which is less likely.
The size of the divided lights in a window is also something that changed quite a bit with time, and some historians have tried using this to estimate the age of old houses. But at least in the case of my house, it proved to be an unreliable method, with their best guesses later proven wrong by hard documentation. The reality is that there has long been a range of what's available, both with regard to size of lights and clarity, for those willing to spend more or less. Seeing a divided-light double-hung window with 8x10 inch panes might cause one to think the house must be 19th century, ignoring the fact that those in the late 18th century could buy panes that large, if they had no care as to the cost.
Moreover, sash could be replaced within existing windows, which may in fact be the case in my house. We know the window frames all date to 1775, and the sash are all identical, making one assume they're also 1775. But it is possible they were all replaced at the same time, re-fitting sash of the same size but fewer and larger panes into the old frames, during a mid-1800's rennovation.
Our original glass is wavy enough that I can tell when there's something or someone in our yard, but I could never actually identify the person without finding a newer replacement pane to peer through. Looking through a few of our waviest windows you could have trouble even telling if it's even a person or a deer!
If anyone wants to argue about glass flowing over time without being heated, read this: